It is often necessary in medical and process instrumentation to provide a small quantity of sample which is to be diluted with a larger quantity of reagent. Measuring an accurate dosage of the two different quantities provides some difficulty.
In some applications, two fluid displacement pumps or syringe pumps have been used to accurately meter small quantities of sample and larger quantities of reagent. In order to obtain very precise measurements, it is preferred not to use a syringe or displacement pump which to meter less than 10% of the volume of the syringe. So in applications where 10 microliters of samples has to be diluted with for example 500 microliters of reagent, two syringes or displacement pumps are needed such as a 100 microliter pump for sample and a 1000 microliter pump for reagent. This leads to duplication of parts and increased expense. Overall size can be larger than would be necessary with the single unit.